Field of the Invention
The present invention relates to methods for the production of solenoidal magnets composed of several axially aligned coils, and solenoidal magnets so produced.
The present invention particularly relates to such solenoidal magnets for use as a magnetic field generator in a Magnetic Resonance Imaging (MRI) system. In particular, the invention relates to such magnets formed of superconductive wire.
Description of the Prior Art
In known magnet arrangements, a solenoidal magnet typically has end coils of relatively large number of turns, and hence larger cross-section and a number of inner coils of smaller number of turns and hence smaller cross-section. Conventionally, an accurately machined former, such as an aluminium tube, is provided with appropriately shaped slots into which wire is wound to form the coils. The coils may be impregnated with a thermosetting resin, either by wet-winding, in which a wire is passed through a bath of resin before being wound onto the former, or the coils may be wound dry, with the completed coils and former later being impregnated in a bath of resin. Similar impregnation may be performed with a wax, but the present description will refer to “resin” only, for brevity.
Alternatively, arrangements of moulded coils are known. In these arrangements, coils are wound into moulds, and the finished coil impregnated with resin within the mould. The resin is then cured, and a solid coil embedded in resin is produced. These moulded coils are then assembled into a magnet, for example by clamping onto a former or other mechanical support structure.
A known compromise arrangement has the inner coils, those toward the axial centre of the magnet, arranged on a former, with end coils moulded and mechanically attached to the former. The end coils tend to be larger in cross-section, and less critical in their placement. This compromise arrangement enables smaller, less expensive formers to be used, while maintaining accurate relative positioning between the inner coils.
These known arrangements suffer from certain drawbacks.
In use, magnet coils are subject to large forces, due to interaction of the coils with the magnetic fields produced. Some of these forces act axially, and urge the coil towards a wall of the former, while other forces act radially, tending to expand the coil to a larger diameter, or compress it onto the former. These forces may cause the coils to move relative to the former. Such movement may cause heating of the coils, which in superconducting magnets may lead to a quench.
The forces acting on the coils may cause the former to flex. The former needs to be large, heavy and mechanically robust to resist those forces. Due to flexure in the former, the force reaction path resisting the coil forces then acts essentially at the inner edges of the coils, which is misaligned from the line of action of the coil body force, which may be considered to act in an axial direction, through the radial mid-point of the coil cross-section. This contributes to a tendency to flex the former. The forces are also borne by a limited surface area of the coils. This may cause deformation of the coils themselves, which may also lead to quench in a superconducting coil.
The majority of the force acts upon the magnet's end coils, and shield coils if present. Inner coils are relatively lightly loaded, but are required to be the most accurately positioned in space to create a homogeneous field as required for imaging.
An accurately-machined former, as conventionally used, is expensive, and is only available from a limited number of suppliers. Transport costs from the former factory to the magnet winding facility may be significant. Storage of the large former may be difficult and costly.
Conventionally, separate coils wound within a former, or individually moulded coils are connected together by wiring and connections made after winding and impregnation of the coils is complete. Much time, and space, has been dedicated to ensuring that these connections are firmly retained in position, and cannot move when the magnet is in operation. A slight movement of these wires may be enough to cause the magnet to quench.